Method for preparing halftone phase shift mask blank, halftone phase shift mask blank, halftone phase shift mask, and thin film forming apparatus

ABSTRACT

A halftone phase shift mask blank comprising a transparent substrate and a halftone phase shift film thereon is prepared through the step of depositing the halftone phase shift film on the substrate by using a sputtering gas containing rare gas and nitrogen gas, and plural targets including at least two silicon targets, applying powers of different values to the silicon targets, effecting reactive sputtering, and rotating the substrate on its axis in a horizontal direction. The halftone phase shift film has satisfactory in-plane uniformity of optical properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of co-pending applicationSer. No. 15/674,709, filed on Aug. 11, 2017, which claims the benefitunder 35 U.S.C. § 119(a) to Patent Application No. 2016-162620, filed inJapan on Aug. 23, 2016, all of which are hereby expressly incorporatedby reference into the present application.

TECHNICAL FIELD

This invention relates to a halftone phase shift mask blank which isprocessed into a halftone phase shift mask for use in themicrofabrication of semiconductor integrated circuits or the like, amethod for preparing the same, a halftone phase shift mask, and anapparatus for forming a thin film for the mask blank.

BACKGROUND ART

In the field of semiconductor technology, research and developmentefforts are continued for further miniaturization of pattern features.Recently, as advances including miniaturization of circuit patterns,thinning of interconnect patterns and miniaturization of contact holepatterns for connection between cell-constituting layers are in progressto comply with higher integration density of LSIs, there is anincreasing demand for the micropatterning technology. Accordingly, inconjunction with the technology for manufacturing photomasks used in theexposure step of the photolithographic microfabrication process, it isdesired to have a technique of forming a more fine and accurate circuitpattern or mask pattern.

In general, reduction projection is employed when patterns are formed onsemiconductor substrates by photolithography. Thus the size of patternfeatures formed on a photomask is about 4 times the size of patternfeatures formed on a semiconductor substrate. In the currentphotolithography technology, the size of circuit patterns printed issignificantly smaller than the wavelength of light used for exposure.Therefore, if a photomask pattern is formed simply by multiplying thesize of circuit pattern 4 times, the desired pattern is not transferredto a resist film on a semiconductor substrate due to opticalinterference and other effects during exposure.

Sometimes, optical interference and other effects during exposure aremitigated by forming the pattern on a photomask to a more complex shapethan the actual circuit pattern. Such a complex pattern shape may bedesigned, for example, by incorporating optical proximity correction(OPC) into the actual circuit pattern. Also, attempts are made to applythe resolution enhancement technology (RET) such as modifiedillumination, immersion lithography or double exposure (or doublepatterning) lithography, to meet the demand for miniaturization andhigher accuracy of patterns.

The phase shift method is used as one of the RET. The phase shift methodis by forming a pattern of film capable of phase reversal ofapproximately 180 degrees on a photomask, such that contrast may beimproved by utilizing optical interference. One of the photomasksadapted for the phase shift method is a halftone phase shift mask.Typically, the halftone phase shift mask includes a substrate of quartzor similar material which is transparent to exposure light, and aphotomask pattern of halftone phase shift film formed on the substrate,capable of providing a phase shift of approximately 180 degrees andhaving an insufficient transmittance to contribute to pattern formation.As the halftone phase shift mask, Patent Document 1 proposes a maskhaving a halftone phase shift film of molybdenum silicide oxide (MoSiO)or molybdenum silicide oxynitride (MoSiON).

For the purpose of forming finer images by photolithography, light ofshorter wavelength is used as the light source. In the currently mostadvanced stage of lithography process, the exposure light source hasmade a transition from KrF excimer laser (248 nm) to ArF excimer laser(193 nm).

CITATION LIST

-   Patent Document 1: JP-A H07-140635-   Patent Document 2: JP-A 2015-111246 (U.S. Pat. No. 9,366,951, EP    2871520)-   Patent Document 3: JP-A 2007-033469-   Patent Document 4: JP-A 2007-233179-   Patent Document 5: JP-A 2007-241065

SUMMARY OF INVENTION

It is known from Patent Document 2 that a halftone phase shift film isimproved in chemical resistance by constructing it from a filmconsisting of silicon, nitrogen and optionally oxygen, for example, atransition metal-free film consisting of silicon and nitrogen, or atransition metal-free film consisting of silicon, nitrogen and oxygen.

In general, a thin film, typically halftone phase shift film, forforming a pattern on a photomask blank is deposited by the sputteringmethod. For example, a film consisting of silicon and nitrogen (SiNfilm) is formed on a transparent substrate by a sputtering process whichinvolves the steps of placing a single silicon target in a depositionchamber, feeding a gas mixture of a rare gas such as argon and nitrogengas to the chamber, applying an electric power to create a gas plasma,and letting the plasma atoms impinge the silicon target to sputtersilicon particles. The thus sputtered silicon particles react withnitrogen on the target surface, take up nitrogen on their way to thesubstrate, or react with nitrogen on the substrate. The resultingsilicon nitride deposits on the substrate. The nitrogen content of theSiN film is controlled by changing the mixing ratio of nitrogen gas inthe gas mixture. The process enables to deposit a SiN film having anydesired nitrogen content on a transparent substrate.

When a halftone phase shift film of silicon nitride is deposited bysputtering a single silicon target, however, film deposition must becarried out by setting the flow rate of nitrogen gas such that the filmmay have desired values of phase shift and transmittance (for example,phase shift median 180±300 and transmittance 3-7%). Due tocharacteristics of reactive sputtering, stable film deposition becomesdifficult in a certain range of nitrogen gas flow rate. Particularly ina region where the nitrogen gas flow rate ranges from a low to moderatelevel (film deposition conditions of transition mode), stable filmdeposition is difficult. In such a flow rate range, a slight variationof nitrogen flow rate leads to a significant variation of filmdeposition state. As a result, the halftone phase shift film has largelyvarying optical properties. In particular, it is difficult to form ahalftone phase shift film having in-plane uniformity of its essentialoptical properties including phase shift and transmittance.

Meanwhile, these problems rarely arise in a region where no nitridingoccurs on the target surface even when nitrogen is introduced and so afilm with a low nitrogen content is deposited (film depositionconditions of metal mode) and a region where the flow rate of nitrogengas is high, the target surface is nitrided, and so a film with a highnitrogen content is deposited (film deposition conditions of reactionmode). Therefore, the problems may be avoided by combining sputteringsteps in these regions to form a halftone phase shift film of multilayerstructure including a low nitrogen content layer and a high nitrogencontent layer. The halftone phase shift film of multilayer structure hashigh in-plane uniformity of optical properties, but is compositionallygraded stepwise in thickness direction. When this halftone phase shiftfilm is processed into a halftone phase shift film pattern, the patternis less vertical in cross-sectional shape.

An object of the invention is to provide a method for preparing ahalftone phase shift mask blank comprising a halftone phase shift filmwherein the halftone phase shift film contains silicon and nitrogen andhas good in-plane uniformity of optical properties, and when thehalftone phase shift film is processed into a halftone phase shift filmpattern, the pattern is fully vertical in cross-sectional shape. Anotherobject is to provide a halftone phase shift mask blank having thehalftone phase shift film, a halftone phase shift mask having a maskpattern of the halftone phase shift film, and an apparatus for forming athin film to construct the mask blank.

The inventor sought for a halftone phase shift film which is composedmainly of silicon and nitrogen, and consists of a single layer whosecomposition is kept constant or continuously graded in thicknessdirection, or two layers including the single layer and a surfaceoxidized layer disposed on a side of the single layer remote from thesubstrate. The halftone phase shift film can be effectively deposited ona surface of a transparent substrate by using a sputtering gascontaining a rare gas and a nitrogen-containing gas, and plural targets,applying powers of different values to the plural targets, effectingreactive sputtering, and rotating the substrate on its axis in ahorizontal direction. When this halftone phase shift film is processedinto a halftone phase shift film pattern, the pattern is fully verticalin cross-sectional shape. While the halftone phase shift film consistsof a single layer or two layers including the single layer and a surfaceoxidized layer, the film provides a phase shift and transmittancerelative to the wavelength of ArF excimer laser, the phase shift havinga median in film plane of 180±300 and the transmittance having a medianin film plane of 3 to 17%. Even in the case of reactive sputtering underfilm deposition conditions of transition mode where a film is depositedin a region of reactive gas flow rate ranging from a low to moderatelevel, there is obtained a halftone phase shift film having highin-plane uniformity of optical properties including phase shift andtransmittance. The invention is predicated on these findings.

The invention provides a method for preparing a halftone phase shiftmask blank, halftone phase shift mask blank, halftone phase shift mask,and apparatus for forming a thin film to construct the mask blank, asdefined below.

In one aspect, the invention provides a method for preparing a halftonephase shift mask blank comprising a transparent substrate and a halftonephase shift film thereon, the halftone phase shift film being composedmainly of silicon and nitrogen, consisting of a single layer whosecomposition is kept constant or continuously graded in thicknessdirection, or two layers including the single layer and a surfaceoxidized layer disposed on a side of the single layer remote from thesubstrate, providing a phase shift relative to the wavelength of ArFexcimer laser, the phase shift having a median in film plane of 180±30°and a difference between maximum and minimum in film plane of up to 2°,and having a transmittance relative to the wavelength of ArF excimerlaser, the transmittance having a median in film plane of 3 to 17% and adifference between maximum and minimum in film plane of up to 0.2%,

the method comprising the step of depositing the halftone phase shiftfilm on a surface of the substrate by using a sputtering gas containinga rare gas and a nitrogen-containing gas, and plural targets includingat least two silicon targets, applying powers of at least two differentvalues to the at least two silicon targets, effecting reactivesputtering, and rotating the substrate on its axis in a horizontaldirection.

In a preferred embodiment, the deposition step includes reactivesputtering in transition mode so that the halftone phase shift film isformed of an unsaturated silicon compound.

Preferably in the sputter deposition step, the plural targets arearranged such that provided that the axis of rotation of the substrateand a vertical line passing the center of a sputter surface of each ofthe plural targets are parallel and spaced apart a distance, one targethas the closest distance between the rotational axis and the verticalline: the distance between the rotational axis and the vertical line ofanother target is 1 to 3 times the distance between the rotational axisand the vertical line of the one target, and the angle included betweennormal lines extending from the rotational axis to vertical lines has amaximum value of 70° to 180°.

In a preferred embodiment, at least two normal lines extend from therotational axis to vertical lines, and any of the angles includedbetween adjacent normal lines is in a range of 70° to 180°.

Most preferably, the plural targets are silicon targets.

In a preferred embodiment, the single layer has a total content ofsilicon and nitrogen of at least 98 at %, and the surface oxidized layercontains silicon, nitrogen and oxygen.

In another aspect, the invention provides a halftone phase shift maskblank comprising a transparent substrate and a halftone phase shift filmthereon,

the halftone phase shift film being composed mainly of silicon andnitrogen, consisting of a single layer whose composition is keptconstant or continuously graded in thickness direction, or two layersincluding the single layer and a surface oxidized layer disposed on aside of the single layer remote from the substrate, providing a phaseshift relative to the wavelength of ArF excimer laser, the phase shifthaving a median in film plane of 180±30° and a difference betweenmaximum and minimum in film plane of up to 2°, and having atransmittance relative to the wavelength of ArF excimer laser, thetransmittance having a median in film plane of 3 to 17% and a differencebetween maximum and minimum in film plane of up to 0.2%.

In a preferred embodiment, the single layer has a total content ofsilicon and nitrogen of at least 98 at %, and the surface oxidized layercontains silicon, nitrogen and oxygen.

In a further aspect, the invention provides a halftone phase shift maskcomprising a transparent substrate and a mask pattern of the halftonephase shift film in the halftone phase shift mask blank defined above.

In a still further aspect, the invention provides an apparatus forforming a thin film to constitute a photomask blank, comprising asubstrate to constitute the photomask blank, plural targets, a gassupply for supplying a sputtering gas containing a rare gas and anitrogen-containing gas, and means for causing electric discharge to theplural targets at the same time, wherein the thin film to constitute aphotomask blank is formed by rotating the substrate on its axis,sputtering the plural targets, and depositing a thin film on thesubstrate, the plural targets are disposed such that provided that therotational axis of the substrate and a vertical line passing the centerof a sputter surface of each of the plural targets are parallel andspaced apart a distance, one target has the closest distance between therotational axis and the vertical line, the distance between therotational axis and the vertical line of another target is 1 to 3 timesthe distance between the rotational axis and the vertical line of theone target, and the angle included between normal lines extending fromthe rotational axis to vertical lines has a maximum value of 70° to180°.

In a preferred embodiment, at least two normal lines extend from therotational axis to vertical lines, and any of the angles includedbetween adjacent normal lines is in a range of 700 to 180°.

Most preferably, the plural targets are silicon targets.

In a preferred embodiment, a halftone phase shift film formed of anunsaturated silicon compound is deposited on the substrate by reactivesputtering in transition mode.

Advantageous Effects of Invention

The halftone phase shift film containing silicon and nitrogen haschemical resistance and good in-plane uniformity of optical properties.When the halftone phase shift film is processed into a halftone phaseshift film pattern, the pattern is fully vertical in cross-sectionalshape. There are obtained a halftone phase shift mask blank comprisingthe halftone phase shift film, and a halftone phase shift maskcomprising a mask pattern of the halftone phase shift film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an elevational view showing one exemplary arrangement of atransparent substrate and targets.

FIG. 2A is a plan view showing one exemplary arrangement of atransparent substrate and targets. FIG. 2B is a plan view showinganother arrangement of a transparent substrate and targets.

FIG. 3A is a plan view showing a further arrangement of a transparentsubstrate and targets. FIG. 3B is a plan view showing a still furtherarrangement of a transparent substrate and targets.

FIG. 4A is a cross-sectional view of one exemplary halftone phase shiftmask blank. FIG. 4B is a cross-sectional view of one exemplary halftonephase shift mask.

FIGS. 5A, 5B and 5C are cross-sectional views of different exemplaryhalftone phase shift mask blanks.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is directed to a halftone phase shift mask blankcomprising a transparent substrate and a halftone phase shift filmformed thereon. According to the inventive method, the halftone phaseshift film is deposited on the substrate by reactive sputtering. Thesputtering step uses a sputtering gas containing a rare gas and areactive gas. A nitrogen-containing gas is essential as the reactivegas, which reacts with the target material, during deposition of thehalftone phase shift film, to form part of the film. Specifically, achoice may be made of nitrogen gas (N₂ gas) and nitrogen oxide gases(N₂O, NO and NO₂ gases), depending on a particular composition of thehalftone phase shift film. Where an oxygen-free film is to be deposited,nitrogen gas (N₂ gas) is used. The sputtering gas contains helium gas,neon gas or argon gas as the rare gas. Depending on a particularcomposition of the halftone phase shift film, another gas such as oxygengas (O₂ gas) or carbon oxide gases (CO and CO₂ gases) may beadditionally used. The content of nitrogen and the contents of otherlight elements such as oxygen and carbon in the halftone phase shiftfilm may be controlled by adjusting the flow rate of sputtering gas,especially reactive gas and the powers applied to targets (to bedescribed below). Sputtering conditions are adjusted such that the filmdeposition rate may fall in an appropriate range.

For sputtering, a plurality of targets are used. The number of targetsmay be 2, 3 or more. A relatively small number of targets are preferredin view of a particle generating source (that can be introduced by acomplex arrangement of targets in the deposition chamber) and targetexchange operation. Thus the number of targets is typically up to 5.Although powers of the same value may be applied to the plural targets,preferably powers of different values are applied to some or all of theplural targets. Typically powers in a range of 100 to 5,000 W areapplied although the power varies with the area of a target, theconstruction of a power supply, and a target cooling system. When pluraltargets are used, and powers are applied to the targets at the sametime, preferably powers of different values are applied to some or allof the plural targets to provoke sputtering discharges, a halftone phaseshift film deposited is improved in the in-plane distribution of opticalproperties over the use of a single target.

The plural targets should include at least two silicon targets (Sitargets, i.e., targets consisting of Si), and preferably all are silicontargets. To the plural silicon targets, powers of different values areapplied. Specifically, when two silicon targets are used, powers ofdifferent values are applied thereto. When three or more silicon targetsare used, powers of mutually different values may be applied, or a powerof identical value may be applied to some targets and a power ofdifferent value may be applied to the remaining target (or powers ofdifferent values may be applied to the remaining targets).

Where a target other than the silicon targets is used, which means thatthree or more targets are used in total, the other target may be asilicon-containing target such as a silicon nitride target or a targetcontaining both silicon and silicon nitride, depending on a particularcomposition of the halftone phase shift film. Then, a transitionmetal-free halftone phase shift film, specifically a halftone phaseshift film of silicon base material such as SiN, SiNO, SiNOC or SiNC maybe formed.

Also, depending on a particular composition of the halftone phase shiftfilm, a target containing a transition metal (Me) such as molybdenum,tungsten, tantalum or zirconium may be used along with asilicon-containing target. Exemplary of the transition metal-containingtarget are a transition metal target and transition metal-silicontargets. Then, a transition metal-containing halftone phase shift film,specifically a halftone phase shift film of transition metal-siliconbase material such as MeSiN, MeSiNO, MeSiNOC or MeSiNC may be formed.

In the sputtering process, particles are sputtered or dislodged from asurface of a target upon application of a power across the target tocause electric discharge, and then deposited onto a surface of arecipient, which is opposed to the target, such as a transparentsubstrate or photomask blank-constructing substrate. As used herein, thetarget surface is referred to as “sputter surface” and the recipientsurface is referred to as “receptive surface.” The size of a target,typically the size of sputter surface of a target is generally adiameter of 100 to 300 mm, when sputter deposition is to a transparentsubstrate of 6 inches squares and 25 mil thick, known as 6025 substrate,as prescribed in the SEMI standards. All targets may be of an identicalsize, or some or all targets may be of different sizes.

In the practice of the invention, since powers of at least two differentvalues are applied to silicon targets, it is essential to performsputter deposition while keeping the receptive surface of a transparentsubstrate horizontal and rotating the substrate on its axis in order toensure the uniformity of sputter deposition on various portions of thesubstrate. The preferred setting is such that the axis of rotationpasses the transparent substrate at its center because the sputterregion may be minimized and the uniformity of sputter deposition beenhanced. During the sputtering process, the substrate may be furtherhorizontally revolved, horizontally oscillated or vertically moved upand down. The sputtering process may be either DC or RF sputtering. Thesputtering pressure is typically 0.01 to 1 Pa, preferably 0.03 to 0.2Pa.

The step of sputter deposition of a halftone phase shift film preferablyincludes reactive sputtering of transition mode such that the halftonephase shift film may be formed of an unsaturated silicon compound. Asused herein, the “transition mode” refers to the state that particlessputtered or dislodged from the target become a silicon compoundsatisfying the unsaturated composition (i.e., A or B value in excessof 1) and form a film on a transparent substrate, and is generallyavailable in a low-to-moderate region of reactive gas flow rate. For thesputter deposition of transition mode, it is necessary to set differentconditions, depending on the type of targets and the type of sputteringgas, especially reactive gas. Typically, the sputter deposition oftransition mode is achievable by adjusting the powers applied to targetsand the flow rate of sputtering gas, especially reactive gas tolow-to-moderate ranges.

The “reaction mode” refers to the state that particles sputtered ordislodged from the target become a silicon compound satisfying thesaturated composition (i.e., A or B value=1) and form a film on atransparent substrate, and is generally available in a high region ofreactive gas flow rate. For the sputter deposition of reaction mode, itis necessary to set different conditions, depending on the type oftargets and the type of sputtering gas, especially reactive gas.Typically, the sputter deposition of reaction mode is achievable byadjusting the powers applied to targets and the flow rate of sputteringgas, especially reactive gas to high ranges. The “metal mode” refers tothe state that particles sputtered or dislodged from the target remainin metal state (metal inclusive of silicon) not having reacted withreactive gas and form a film on a transparent substrate. The sputterdeposition of metal mode is achievable by adjusting the flow rate ofreactive gas to a low range, especially omitting the reactive gas (flowrate of reactive gas=0).

Whether the sputter deposition state is of transition mode, reactionmode or metal mode may be confirmed by actually performing sputteringunder predetermined conditions and analyzing the composition of theresulting film. It is noted that when two or more targets are used, insome cases, all the targets may take the transition mode, and in caseswhere the powers applied to the targets are different the targets maytake different modes. For example, even under deposition conditionswhere the flow rate of reactive gas is identical, if the power appliedto a target is high, the amount of metal sputtered is large, andreaction does not proceed, resulting in the transition mode of releasingunsaturated silicon compound; and if the power applied to a target islow, the amount of metal sputtered is small, and reaction proceeds,resulting in the reaction mode of releasing saturated silicon compound.When a single layer film having a constant composition in thicknessdirection is formed, it suffices that at least one target is sputteredin transition mode. When one or more targets are sputtered in transitionmode, the remaining target(s) may be sputtered in either reaction ormetal mode.

When a single layer film having a composition continuously graded inthickness direction is formed, the powers applied to targets and theflow rate of sputtering gas, especially reactive gas are changedcontinuously or stepwise, i.e., in many steps of short intervals. Inthese cases, it suffices that at least one target is sputtered intransition mode, during part or the entirety of sputtering process. Whenone or more targets are sputtered in transition mode, the remainingtarget(s) may be sputtered in either reaction or metal mode, and in thiscase, the sputtering process may pass the reaction or metal mode.

When a single layer film having a constant composition in thicknessdirection is formed, and when a single layer film having a compositioncontinuously graded in thickness direction is formed, it is moreeffective in view of in-plane uniformity of optical properties of thehalftone phase shift film that all targets are sputtered in transitionmode.

It is assumed that the substrate has an axis of rotation, a verticalline passes the center of a sputter surface of each of the pluraltargets, the rotational axis and the vertical line are parallel andspaced apart a distance, and one target has the closest distance betweenthe rotational axis and the vertical line. The plural targets arepreferably arranged such that the distance between the rotational axisand the vertical line of another target or targets is 1 to 3 times, morepreferably 1 to 2 times, and even more preferably 1 to 1.5 times the(closest) distance between the rotational axis and the vertical line ofthe one target. More preferably, all the targets are arranged on any oftwo, three or more concentric circles about the rotational axis of thesubstrate. Most preferably plural targets are arranged such that thedistance between the rotational axis and the vertical line of anothertarget or targets is equal to the distance between the rotational axisand the vertical line of the one target (having the closest distancebetween the rotational axis and the vertical line), that is, all targetsare arranged on one concentric circle about the rotational axis of thesubstrate.

On the assumption that the substrate has an axis of rotation and avertical line passes the center of a sputter surface of each of theplural targets, the plural targets are preferably arranged such that theangle included between normal lines extending from the rotational axisto vertical lines has a maximum value of 70° to 180°. In a preferredembodiment, the plural targets are arranged such that at least twonormal lines extend from the rotational axis to vertical lines, and anyof the angles included between adjacent normal lines ranges from 70°,especially from 120°, to 180°, preferably to less than 180°, andespecially to 140°.

The above-mentioned arrangement of plural targets is effective in a thinfilm-forming system (sputtering system) wherein a thin film such ashalftone phase shift film is deposited on a recipient (e.g., photomaskblank-constructing substrate such as transparent substrate) bysputtering plural targets while rotating the recipient on its axis ofrotation. The thin film-forming system preferably include a sputterchamber, a gas supply for supplying a sputtering gas containing a raregas such as argon gas and a nitrogen-containing gas such as nitrogengas, and means for provoking electric discharges to the plural targetsat the same time. The means is preferably capable of applying electricpowers to the plural targets at the same time, preferably applyingelectric powers of different values to some or all targets, therebyprovoking sputtering discharges.

Now referring to the accompanying figures, the axis of rotation of thetransparent substrate, vertical lines extending through the targets, thedistance between the rotational axis and the vertical line, normal linesextending from the rotational axis of the substrate to the verticallines through the targets, the direction of the normal line, and theangle between normal lines are described. FIG. 1 is an elevational viewshowing one exemplary arrangement of a transparent substrate S andtargets T. The substrate S has a receptive surface SS which is kepthorizontal, and a rotational axis “a” passing the center of substrate Sand extending perpendicular to the receptive surface SS (i.e., invertical or gravity direction). On the other hand, each target T has asputter surface TS, and a vertical line “v” passing the center of thesputter surface TS of target T and extending perpendicular to thereceptive surface SS of the substrate (i.e., in vertical or gravitydirection). The sputter surface TS of target T is parallel to thereceptive surface SS of substrate S, that is, kept horizontal. Thetargets may be inclined as long as the sputter surface of each targetfaces the receptive surface of the substrate. In this case too, thevertical line passes the center of the sputter surface of each target.The distance “d” between the vertical line “v” through target T and therotational axis “a” of substrate S is equal to the distance betweenrotational axis “a” and vertical line “v” on a normal line “n”.

FIGS. 2A, 2B, 3A and 3B are plan views showing exemplary arrangements oftransparent substrate S and targets T. For each target T, a normal line“n” extending from the rotational axis of substrate S to a vertical linethrough the target and the direction of the normal line are provided.The targets are arranged such that the maximum of angles between normallines falls in the predetermined range, and more preferably there arethree or more directions of normal lines to vertical lines and theangles included between any two adjacent normal lines fall within thepredetermined range. In this case too, the distance “d” between thevertical line (not shown) through target T and the rotational axis (notshown) of substrate S is equal to the distance between the rotationalaxis and the vertical line on normal line “n”. Specifically, referringto FIG. 2A, the angle θ included between normal lines “n” associatedwith two targets T is maximum and falls within the predetermined range,and there are two directions of normal lines “n” to vertical linesthrough the targets, and the angle included between the directions of apair of adjacent normal lines “n” falls within the predetermined range.

In the arrangement of FIG. 2B, the maximum θ3 among the three angles θ1,θ2 and θ3 included between normal lines “n” associated with threetargets T falls within the predetermined range. In the arrangement ofFIG. 3(A), the maximum θ3 among the three angles θ1, θ2 and θ3 includedbetween normal lines “n” associated with three targets T falls withinthe predetermined range. There are three directions of normal lines “n”to vertical lines through the targets, and the angles θ1, θ2 and θ3included between the directions of three pairs of adjacent normal lines“n” fall within the predetermined range.

In the arrangement of FIG. 3B, the maximum θ2, θ3 among the three anglesθ1, θ2 and θ3 included between normal lines “n” associated with threetargets T falls within the predetermined range (specifically, θ2=θ3, andθ1=0°), and there are two directions of normal lines “n” to verticallines through the targets, and the angle θ2, θ3 included between thedirections of a pair of adjacent normal lines “n” falls within thepredetermined range. Where three or more targets are used, the targetsmay be arranged such that all normal lines associated therewith are indifferent directions, or normal lines associated with two or moretargets are in an identical direction, and the direction of normal lineassociated with another target is different from the overlapped normalline direction. In the latter embodiment wherein the targets have theoverlapped normal line, the vertical lines through these targets arealigned with the overlapped normal line.

As discussed above, targets are arranged such that the maximum of theangles included between normal lines extending from the rotational axisof the substrate to the vertical lines through the targets falls in thepredetermined range, and targets having different directions of normallines thereto are arranged such that a predetermined angle is includedbetween the directions of adjacent normal lines. Then a halftone phaseshift film having improved in-plane uniformity of optical properties isobtained. If targets are arranged in one direction, spots of lowreactive gas concentration are locally created during sputtering,leading to low uniformity of reaction within the sputtering space. Thatis, variations occur in the progress of reaction of target metal andreactive gas at any of the sputter surface of targets, sputteredparticles, and the receptive surface of substrate. The arrangement oftargets according to the invention ensures the uniformity of reaction oftarget metal with reactive gas within the sputtering space, andeventually enhances the in-plane uniformity of optical properties of theresulting halftone phase shift film.

The halftone phase shift mask blank of the invention is defined ascomprising a transparent substrate, typically quartz substrate and ahalftone phase shift film disposed thereon directly or via another film.Preference is given to transparent substrates of 6 inch squares and 25mil thick, known as 6025 substrate, as prescribed in the SEMI standards,or transparent substrates of 152 mm squares and 6.35 mm thick whenexpressed in the SI units. The halftone phase shift mask has a(photo)mask pattern of the halftone phase shift film.

The halftone phase shift film is composed mainly of silicon andnitrogen. In one embodiment, the film consists of a single layer whosecomposition is kept constant or continuously graded in thicknessdirection. The continuously graded composition means that thecomposition changes so little that no inflection point is detected oncompositional analysis. Since sputtering characteristics suggest amoderate response of a film composition change to changes of sputteringconditions, a continuous grading of composition may be obtained bychanging sputtering conditions such as the powers applied to the targetsand the flow rate of sputtering gas, especially reactive gas, in manysteps of short intervals. Preferably a continuous grading of compositionis obtained by continuously changing sputtering conditions.

FIG. 4A is a cross-sectional view of a halftone phase shift mask blankin one embodiment of the invention. The halftone phase shift mask blank100 includes a transparent substrate 10 and a halftone phase shift film1 formed thereon. FIG. 4B is a cross-sectional view of a halftone phaseshift mask in one embodiment of the invention. The halftone phase shiftmask 101 includes a transparent substrate 10 and a halftone phase shiftfilm pattern 11 formed thereon.

In the embodiment wherein the halftone phase shift film is a singlelayer film, it contains silicon and nitrogen. Preferably the film iscomposed mainly of silicon and nitrogen. Specifically a total content ofsilicon and nitrogen is at least 90 at %, more preferably at least 95 at%, and even more preferably at least 98 at %. The silicon content ispreferably at least 35 at %, more preferably at least 42 at % and up to50 at %, more preferably up to 48 at %. The nitrogen content ispreferably at least 45 at %, more preferably at least 50 at % and up to57 at %, more preferably up to 55 at %. In addition to silicon andnitrogen, the single layer film that constitutes the halftone phaseshift film may contain a transition metal such as molybdenum, tungsten,tantalum or zirconium, especially molybdenum. In this case, the contentof transition metal is preferably up to 5 at %. Besides nitrogen, thefilm may contain a light element such as oxygen or carbon, and in thiscase, the content of light element (other than nitrogen) is preferablyup to 5 at %. Preferably the single layer film that constitutes thehalftone phase shift film consists of silicon, nitrogen and oxygen, mostpreferably consists of silicon and nitrogen.

The halftone phase shift film may include a surface oxidized layer asthe surface-side layer (or outermost layer) in order to suppress anychange in quality of the film. Specifically, the halftone phase shiftfilm may be constructed by two layers, a single layer film composedmainly of silicon and nitrogen and having a composition which is keptconstant or continuously graded in thickness direction and a surfaceoxidized layer disposed on a side of the single layer film remote fromthe substrate. The thickness of the surface oxidized layer is preferablyup to 10%, more preferably up to 5% of the overall thickness of thehalftone phase shift film. The surface oxidized layer may have an oxygencontent of at least 20 at %, with even an oxygen content of at least 50at % being acceptable. In addition to oxygen, the surface oxidized layerpreferably contains silicon and nitrogen and may further contain atransition metal such as molybdenum, tungsten, tantalum or zirconium, ora light element such as oxygen or carbon. Preferably the surfaceoxidized layer consists of silicon, nitrogen and oxygen.

The surface oxidized layer may be formed by atmospheric or air oxidationor forced oxidative treatment. Examples of forced oxidative treatmentinclude treatment of a silicon-based material film with ozone gas orozone water, and heating of a film at about 300° C. or higher in anoxygen-containing atmosphere, typically oxygen gas atmosphere by ovenheating, lamp annealing or laser heating. The surface oxidized layerpreferably has a thickness of up to 10 nm, more preferably up to 5 nm,and even more preferably up to 3 nm. The oxidized layer exerts itseffect as long as its thickness is at least 1 nm. The surface oxidizedlayer is preferably formed by atmospheric oxidation or oxidativetreatment as mentioned above in order that the layer contains leastdefects.

The halftone phase shift film is preferably formed of an unsaturatedsilicon compound. In the embodiment wherein the halftone phase shiftfilm is a single layer film, the single layer film is preferably formedof an unsaturated silicon compound; in the embodiment wherein thehalftone phase shift film includes the single layer film and a surfaceoxidized layer, one or both of the single layer film and the surfaceoxidized layer, especially at least the single layer film is preferablyformed of an unsaturated silicon compound. On the assumption that thesilicon compound consists of silicon and nitrogen, or silicon, nitrogenand oxygen and/or carbon, silicon is tetravalent, nitrogen is trivalent,oxygen is divalent, and carbon is tetravalent, the term “unsaturation”as used herein refers to a composition wherein the total of equivalentsof nitrogen, oxygen and carbon is less than the equivalent of silicon.Specifically, a composition wherein a value of A which is calculatedfrom the equation:

A=C _(Si)×4/(C _(N)×3+C _(O)×2+C _(C)×4)

wherein C_(Si) is a silicon content, C_(N) is a nitrogen content, C_(O)is an oxygen content, and C_(C) is a carbon content (all in at %) is inexcess of 1 is an unsaturated composition. The unsaturated state mayalso be defined as a silicon compound having a Si—Si bond. Theunsaturated composition may be obtained by sputtering in transition modeduring sputter deposition of a halftone phase shift film.

Where the silicon compound of which the halftone phase shift film isconstructed further contains a transition metal, the term “unsaturation”refers to a composition wherein the total of equivalents of nitrogen,oxygen and carbon is less than the total of equivalents of silicon andtransition metal. For example, where the silicon compound containshexavalent molybdenum as the transition metal, the term “unsaturation”refers to a composition wherein the total of equivalents of nitrogen,oxygen and carbon is less than the total of equivalents of silicon andmolybdenum. Specifically, a composition wherein a value of B which iscalculated from the equation:

B=(C _(Si)×4+C _(Mo)×6)/(C _(N)×3+C _(O)×2+C _(C)×4)

wherein C_(Si) is a silicon content, C_(Mo) is a molybdenum content,C_(N) is a nitrogen content, C_(O) is an oxygen content, and C_(C) is acarbon content (all in at %) is in excess of 1 is an unsaturatedcomposition.

The upper limit of A and B values for unsaturated silicon compounds istypically up to 1.5, especially up to 1.3. In contrast to theunsaturated silicon compounds, silicon compounds whose composition has aA or B value of 1 may be designated saturated silicon compounds. Thesaturated state may also be defined as a silicon compound free of aSi—Si bond. The saturated composition may be obtained by sputtering inreaction mode during sputter deposition of a halftone phase shift film.

The phase shift of the halftone phase shift film with respect toexposure light is such that a phase shift between the exposure lighttransmitted by a region of phase shift film (phase shift region) and theexposure light transmitted by a neighboring region where the phase shiftfilm is removed, causes interference of exposure light whereby contrastis increased. Specifically the phase shift is in a range of 180±30degrees, i.e., from 150° to 210°. Although ordinary halftone phase shiftfilms are set to a phase shift of approximately 180°, it is possiblefrom the standpoint of contrast enhancement to adjust the phase shiftbelow or beyond 180° rather than limiting to approximately 180°. Forexample, setting a phase shift of smaller than 180° is effective forforming a thinner film. It is a matter of course that a phase shiftcloser to 180° is more effective because a higher contrast is available.In this regard, the phase shift is preferably 160 to 190°, morepreferably 175 to 185°. For the halftone phase shift film, the median ofin-plane phase shift should preferably fall in the above-defined range.Further the difference between maximum and minimum of in-plane phaseshift of the halftone phase shift film should preferably be up to 2°,especially up to 1°.

The halftone phase shift film has a transmittance of exposure lightwhich is preferably at least 3%, more preferably at least 5%, and up to17%, more preferably up to 12%. For the halftone phase shift film, themedian of in-plane transmittance should preferably fall in theabove-defined range. Further the difference between maximum and minimumof in-plane transmittance should preferably be up to 0.2%, especially upto 0.1%.

The overall thickness of the halftone phase shift film should preferablybe up to 70 nm, more preferably up to 65 nm, because a thinner filmfacilitates to form a finer pattern. The overall thickness of thehalftone phase shift film is typically at least 58 nm as lower limit.

In the halftone phase shift mask blank of the invention, a second filmof single layer or multilayer structure may be formed on the halftonephase shift film. Most often, the second film is disposed contiguous tothe halftone phase shift film. Examples of the second film include alight-shielding film, a combination of light-shielding film andantireflective film, and an auxiliary film which functions as a hardmask during subsequent pattern formation of the halftone phase shiftfilm. When a third film is formed as will be described later, the secondfilm may be utilized as an auxiliary film (etching stop film) whichfunctions as an etching stopper during subsequent pattern formation ofthe third film. The second film is preferably composed of achromium-containing material.

One exemplary embodiment is a halftone phase shift mask blankillustrated in FIG. 5A. The halftone phase shift photomask blankdepicted at 100 in FIG. 5A includes a transparent substrate 10, ahalftone phase shift film 1 formed on the substrate, and a second film 2formed on the film 1.

The halftone phase shift mask blank may include a light-shielding filmas the second film on the halftone phase shift film. A combination of alight-shielding film and an antireflective film may also be used as thesecond film. The provision of the second film including alight-shielding film ensures that a halftone phase shift mask includes aregion capable of completely shielding exposure light. Thelight-shielding film and antireflective film may also be utilized as anauxiliary film during etching. The construction and material of thelight-shielding film and antireflective film are known from many patentdocuments, for example, Patent Document 3 (JP-A 2007-033469) and PatentDocument 4 (JP-A 2007-233179). One preferred film construction of thelight-shielding film and antireflective film is a structure having alight-shielding film of Cr-containing material and an antireflectivefilm of Cr-containing material for reducing reflection by thelight-shielding film. Each of the light-shielding film and theantireflective film may be a single layer or multilayer. SuitableCr-containing materials of which the light-shielding film andantireflective film are made include chromium alone, chromium oxide(CrO), chromium nitride (CrN), chromium carbide (CrC), chromiumoxynitride (CrON), chromium oxycarbide (CrOC), chromium nitride carbide(CrNC), chromium oxynitride carbide (CrONC) and other chromiumcompounds.

Where the second film is a light-shielding film or a combination of alight-shielding film and an antireflective film, the light-shieldingfilm is made of a chromium-based material having a chromium content ofat least 40 at %, especially at least 60 at % and less than 100 at %,preferably up to 99 at %, and more preferably up to 90 at %. Thechromium-based material has an oxygen content of at least 0 at % and upto 60 at %, preferably up to 40 at %, with an oxygen content of at least1 at % being preferred when an etching rate must be adjusted. Thechromium-based material has a nitrogen content of at least 0 at % and upto 50 at %, preferably up to 40 at %, with a nitrogen content of atleast 1 at % being preferred when an etching rate must be adjusted. Thechromium-based material has a carbon content of at least 0 at % and upto 20 at %, preferably up to 10 at %, with a carbon content of at least1 at % being preferred when an etching rate must be adjusted. The totalcontent of chromium, oxygen, nitrogen and carbon is preferably at least95 at %, more preferably at least 99 at %, and especially 100 at %.

Where the second film is a combination of a light-shielding film and anantireflective film, the antireflective film is preferably made of achromium-containing material having a chromium content of preferably atleast 30 at %, more preferably at least 35 at % and preferably up to 70at %, and more preferably up to 50 at %. The chromium-containingmaterial preferably has an oxygen content of up to 60 at %, and at least1 at % and more preferably at least 20 at %. The chromium-containingmaterial preferably has a nitrogen content of up to 50 at %, morepreferably up to 30 at %, and at least 1 at %, more preferably at least3 at %. The chromium-containing material preferably has a carbon contentof at least 0 at % and up to 20 at %, more preferably up to 5 at %, witha carbon content of at least 1 at % being preferred when an etching ratemust be adjusted. The total content of chromium, oxygen, nitrogen andcarbon is preferably at least 95 at %, more preferably at least 99 at %,and especially 100 at %.

Where the second film is a light-shielding film or a combination of alight-shielding film and an antireflective film, the second film has athickness of typically 20 to 100 nm, preferably 40 to 70 nm. Also thehalftone phase shift film combined with the second film shouldpreferably have a total optical density of at least 2.0, more preferablyat least 2.5, and even more preferably at least 3.0, with respect toexposure light of wavelength up to 200 nm.

In the halftone phase shift mask blank of the invention, a third film ofsingle layer or multilayer structure may be formed on the second film.Most often, the third film is disposed contiguous to the second film.Examples of the third film include a light-shielding film, a combinationof light-shielding film and antireflective film, and an auxiliary filmwhich functions as a hard mask during subsequent pattern formation ofthe second film. The third film is preferably composed of asilicon-containing material, especially chromium-free silicon-containingmaterial.

One exemplary embodiment is a halftone phase shift mask blankillustrated in FIG. 5B. The halftone phase shift mask blank depicted at100 in FIG. 5B includes a transparent substrate 10, a halftone phaseshift film 1 formed on the substrate, a second film 2 formed on the film1, and a third film 3 formed on the second film 2.

Where the second film is a light-shielding film or a combination of alight-shielding film and an antireflective film, the third film may bean auxiliary film (etching mask film) which functions as a hard maskduring subsequent pattern formation of the second film. When a fourthfilm is formed as will be described later, the third film may beutilized as an auxiliary film (etching stop film) which functions as anetching stopper during subsequent pattern formation of the fourth film.The auxiliary film is preferably composed of a material having differentetching properties from the second film, for example, a material havingresistance to chlorine dry etching applied to the etching ofchromium-containing material, specifically a silicon-containing materialwhich can be etched with fluoride gas such as SF₆ or CF₄. Suitablesilicon-containing materials include silicon alone, a materialcontaining silicon and one or both of nitrogen and oxygen, a materialcontaining silicon and a transition metal, and a material containingsilicon, one or both of nitrogen and oxygen, and a transition metal.Exemplary of the transition metal are molybdenum, tantalum andzirconium.

Where the third film is an auxiliary film, it is preferably composed ofa silicon-containing material having a silicon content of preferably atleast 20 at %, more preferably at least 33 at % and up to 95 at %, morepreferably up to 80 at %. The silicon-containing material has a nitrogencontent of at least 0 at % and up to 50 at %, preferably up to 30 at %,with a nitrogen content of at least 1 at % being preferred when anetching rate must be adjusted. The silicon-containing material has anoxygen content of at least 0 at %, preferably at least 20 at % and up to70 at %, preferably up to 66 at %, with an oxygen content of at least 1at % being preferred when an etching rate must be adjusted. Thesilicon-containing material has a transition metal content of at least 0at % and up to 35 at %, preferably up to 20 at %, with a transitionmetal content of at least 1 at % being preferred if present. The totalcontent of silicon, oxygen, nitrogen and transition metal is preferablyat least 95 at %, more preferably at least 99 at %, and especially 100at %.

Where the second film is a light-shielding film or a combination of alight-shielding film and an antireflective film and the third film is anauxiliary film, the second film has a thickness of typically 20 to 100nm, preferably 40 to 70 nm, and the third film has a thickness oftypically 1 to 30 nm, preferably 2 to 15 nm. Also the halftone phaseshift film combined with the second film should preferably have a totaloptical density of at least 2.0, more preferably at least 2.5, and evenmore preferably at least 3.0, with respect to exposure light ofwavelength up to 200 nm.

Where the second film is an auxiliary film, a light-shielding film maybe formed as the third film. Also, a combination of a light-shieldingfilm and an antireflective film may be formed as the third film. Hereinthe second film may be utilized as an auxiliary film (etching mask film)which functions as a hard mask during pattern formation of the halftonephase shift film, or an auxiliary film (etching stop film) whichfunctions as an etching stopper during pattern formation of the thirdfilm. Examples of the auxiliary film are films of chromium-containingmaterials as described in Patent Document 5 (JP-A 2007-241065). Theauxiliary film may be a single layer or multilayer. Suitablechromium-containing materials of which the auxiliary film is madeinclude chromium alone, chromium oxide (CrO), chromium nitride (CrN),chromium carbide (CrC), chromium oxynitride (CrON), chromium oxycarbide(CrOC), chromium nitride carbide (CrNC), chromium oxynitride carbide(CrONC) and other chromium compounds.

Where the second film is an auxiliary film, the film preferably has achromium content of preferably at least 40 at %, more preferably atleast 50 at % and up to 100 at %, more preferably up to 99 at %, andeven more preferably up to 90 at %. The film has an oxygen content of atleast 0 at %, and up to 60 at %, preferably up to 55 at %, with anoxygen content of at least 1 at % being preferred when an etching ratemust be adjusted. The film has a nitrogen content of at least 0 at %,and up to 50 at %, preferably up to 40 at %, with a nitrogen content ofat least 1 at % being preferred when an etching rate must be adjusted.The film has a carbon content of at least 0 at % and up to 20 at %,preferably up to 10 at %, with a carbon content of at least 1 at % beingpreferred when an etching rate must be adjusted. The total content ofchromium, oxygen, nitrogen and carbon is preferably at least 95 at %,more preferably at least 99 at %, and especially 100 at %.

On the other hand, the light-shielding film and antireflective film asthe third film is preferably composed of a material having differentetching properties from the second film, for example, a material havingresistance to chlorine dry etching applied to the etching ofchromium-containing material, specifically a silicon-containing materialwhich can be etched with fluoride gas such as SF₆ or CF₄. Suitablesilicon-containing materials include silicon alone, a materialcontaining silicon and one or both of nitrogen and oxygen, a materialcontaining silicon and a transition metal, and a material containingsilicon, one or both of nitrogen and oxygen, and a transition metal.Exemplary of the transition metal are molybdenum, tantalum andzirconium.

Where the third film is a light-shielding film or a combination of alight-shielding film and an antireflective film, the light-shieldingfilm and antireflective film are preferably composed of asilicon-containing material having a silicon content of preferably atleast 10 at %, more preferably at least 30 at % and less than 100 at %,more preferably up to 95 at %. The silicon-containing material has anitrogen content of at least 0 at/o and up to 50 at %, preferably up to40 at %, especially up to 20 at %, with a nitrogen content of at least 1at % being preferred when an etching rate must be adjusted. Thesilicon-containing material has an oxygen content of at least 0 at %,and up to 60 at %, preferably up to 30 at %, with an oxygen content ofat least 1 at % being preferred when an etching rate must be adjusted.The silicon-containing material has a transition metal content of atleast 0 at % and up to 35 at %, preferably up to 20 at %, with atransition metal content of at least 1 at % being preferred if present.The total content of silicon, oxygen, nitrogen and transition metal ispreferably at least 95 at %, more preferably at least 99 at %, andespecially 100 at %.

Where the second film is an auxiliary film and the third film is alight-shielding film or a combination of a light-shielding film and anantireflective film, the second film has a thickness of typically 1 to20 nm, preferably 2 to 10 nm, and the third film has a thickness oftypically 20 to 100 nm, preferably 30 to 70 nm. Also the halftone phaseshift film combined with the second and third films should preferablyhave a total optical density of at least 2.0, more preferably at least2.5, and even more preferably at least 3.0, with respect to exposurelight of wavelength up to 200 nm.

In the halftone phase shift mask blank of the invention, a fourth filmof single layer or multilayer structure may be formed on the third film.Most often, the fourth film is disposed contiguous to the third film.Exemplary of the fourth film is an auxiliary film which functions as ahard mask during subsequent pattern formation of the third film. Thefourth film is preferably composed of a chromium-containing material.

One exemplary embodiment is a halftone phase shift mask blankillustrated in FIG. 5C. The halftone phase shift mask blank depicted at100 in FIG. 5C includes a transparent substrate 10, a halftone phaseshift film 1 formed on the substrate, a second film 2 formed on the film1, a third film 3 formed on the second film 2, and a fourth film 4formed on the third film 3.

Where the third film is a light-shielding film or a combination of alight-shielding film and an antireflective film, the fourth film may bean auxiliary film (etching mask film) which functions as a hard maskduring subsequent pattern formation of the third film. The auxiliaryfilm is preferably composed of a material having different etchingproperties from the third film, for example, a material havingresistance to fluorine dry etching applied to the etching ofsilicon-containing material, specifically a chromium-containing materialwhich can be etched with oxygen-containing chloride gas. Suitablechromium-containing materials include chromium alone, chromium oxide(CrO), chromium nitride (CrN), chromium carbide (CrC), chromiumoxynitride (CrON), chromium oxycarbide (CrOC), chromium nitride carbide(CrNC), chromium oxynitride carbide (CrONC) and other chromiumcompounds.

Where the fourth film is an auxiliary film, the film has a chromiumcontent of at least 40 at %, preferably at least 50 at % and up to 100at %, preferably up to 99 at %, and more preferably up to 90 at %. Thefilm has an oxygen content of at least 0 at % and up to 60 at %,preferably up to 40 at %, with an oxygen content of at least 1 at %being preferred when an etching rate must be adjusted. The film has anitrogen content of at least 0 at % and up to 50 at %, preferably up to40 at %, with a nitrogen content of at least 1 at % being preferred whenan etching rate must be adjusted. The film has a carbon content of atleast 0 at % and up to 20 at %, preferably up to 10 at %, with a carboncontent of at least 1 at % being preferred when an etching rate must beadjusted. The total content of chromium, oxygen, nitrogen and carbon ispreferably at least 95 at %, more preferably at least 99 at %, andespecially 100 at %.

Where the second film is an auxiliary film, the third film is alight-shielding film or a combination of a light-shielding film and anantireflective film, and the fourth film is an auxiliary film; thesecond film has a thickness of typically 1 to 20 nm, preferably 2 to 10nm, the third film has a thickness of typically 20 to 100 nm, preferably30 to 70 nm, and the fourth film has a thickness of typically 1 to 30nm, preferably 2 to 20 nm. Also the halftone phase shift film combinedwith the second and third films should preferably have a total opticaldensity of at least 2.0, more preferably at least 2.5, and even morepreferably at least 3.0, with respect to exposure light of wavelength upto 200 nm.

The second and fourth films of chromium-containing materials may bedeposited by reactive sputtering using a chromium target or a chromiumtarget having one or more of oxygen, nitrogen and carbon added thereto,and a sputtering gas based on a rare gas such as Ar, He or Ne, to whicha gas selected from oxygen-containing gas, nitrogen-containing gas andcarbon-containing gas is added depending on the desired composition of afilm to be deposited.

The third film of silicon-containing material may be deposited byreactive sputtering using a silicon target, silicon nitride target,target containing silicon and silicon nitride, transition metal target,or composite silicon/transition metal target, and a sputtering gas basedon a rare gas such as Ar, He or Ne, to which a gas selected fromoxygen-containing gas, nitrogen-containing gas and carbon-containing gasis added depending on the desired composition of a film to be deposited.

The halftone phase shift mask blank may be processed into a halftonephase shift mask by a standard technique. For example, a halftone phaseshift photomask blank comprising only a halftone phase shift filmdeposited on a transparent substrate may be processed as follows. First,a resist film adapted for electron beam (EB) lithography is formed onthe halftone phase shift film, exposed to a pattern of EB, and developedin a conventional way, forming a resist pattern. While the resistpattern thus obtained is used as etching mask, fluorine base dry etchingis carried out for transferring the resist pattern to the halftone phaseshift film, obtaining a pattern of the halftone phase shift film. Theresist pattern is removed in a conventional manner, yielding a halftonephase shift mask.

In another example, a halftone phase shift mask blank comprising ahalftone phase shift film and a second film of chromium-containingmaterial deposited thereon may be processed into a mask as follows.First, a resist film adapted for EB lithography is formed on the secondfilm of the halftone phase shift mask blank, exposed to a pattern of EB,and developed in a conventional way, forming a resist pattern. While theresist pattern thus obtained is used as etching mask, oxygen-containingchlorine base dry etching is carried out for transferring the resistpattern to the second film, obtaining a pattern of the second film.Next, while the second film pattern is used as etching mask, fluorinebase dry etching is carried out for transferring the pattern to thehalftone phase shift film, obtaining a pattern of the halftone phaseshift film. If any region of the second film is to be left, a resistpattern for protecting that region is formed on the second film.Thereafter, the portion of the second film which is not protected withthe resist pattern is removed by oxygen-containing chlorine base dryetching. The resist pattern is removed in a conventional manner,yielding a halftone phase shift mask.

In a further example, a halftone phase shift mask blank comprising ahalftone phase shift film, a light-shielding film or a light-shieldingfilm/antireflective film of chromium-containing material depositedthereon as a second film, and an auxiliary film of silicon-containingmaterial deposited thereon as a third film may be processed into a maskas follows. First, a resist film adapted for EB lithography is formed onthe third film of the halftone phase shift photomask blank, exposed to apattern of EB, and developed in a conventional way, forming a resistpattern. While the resist pattern thus obtained is used as etching mask,fluorine base dry etching is carried out for transferring the resistpattern to the third film, obtaining a pattern of the third film. Whilethe third film pattern thus obtained is used as etching mask,oxygen-containing chlorine base dry etching is carried out fortransferring the third film pattern to the second film, obtaining apattern of the second film. The resist pattern is removed at this point.Further, while the second film pattern is used as etching mask, fluorinebase dry etching is carried out for transferring the second film patternto the halftone phase shift film to define a pattern of the halftonephase shift film and at the same time, removing the third film pattern.If any region of the second film is to be left, a resist pattern forprotecting that region is formed on the second film. Thereafter, theportion of the second film which is not protected with the resistpattern is removed by oxygen-containing chlorine base dry etching. Theresist pattern is removed in a conventional manner, yielding a halftonephase shift mask.

In a still further example, a halftone phase shift mask blank comprisinga halftone phase shift film, an auxiliary film of chromium-containingmaterial deposited thereon as a second film, and a light-shielding filmor a light-shielding film/antireflective film of silicon-containingmaterial deposited on the second film as a third film may be processedinto a mask as follows. First, a resist film adapted for EB lithographyis formed on the third film of the halftone phase shift mask blank,exposed to a pattern of EB, and developed in a conventional way, forminga resist pattern. While the resist pattern thus obtained is used asetching mask, fluorine base dry etching is carried out for transferringthe resist pattern to the third film, obtaining a pattern of the thirdfilm. While the third film pattern thus obtained is used as etchingmask, oxygen-containing chlorine base dry etching is carried out fortransferring the third film pattern to the second film, whereby apattern of the second film is obtained, that is, a portion of the secondfilm where the halftone phase shift film is to be removed is removed.The resist pattern is removed at this point. A resist pattern forprotecting a portion of the third film to be left is formed on the thirdfilm. Further, while the second film pattern is used as etching mask,fluorine base dry etching is carried out for transferring the secondfilm pattern to the halftone phase shift film to define a pattern of thehalftone phase shift film and at the same time, removing the portion ofthe third film which is not protected with the resist pattern. Theresist pattern is removed in a conventional manner. Finally,oxygen-containing chlorine base dry etching is carried out to remove theportion of the second film where the third film has been removed,yielding a halftone phase shift mask.

In a still further example, a halftone phase shift mask blank comprisinga halftone phase shift film, an auxiliary film of chromium-containingmaterial deposited thereon as a second film, a light-shielding film or alight-shielding film/antireflective film of silicon-containing materialdeposited on the second film as a third film, and an auxiliary film ofchromium-containing material deposited on the third film as a fourthfilm may be processed into a mask as follows. First, a resist filmadapted for EB lithography is formed on the fourth film of the halftonephase shift mask blank, exposed to a pattern of EB, and developed in aconventional way, forming a resist pattern. While the resist patternthus obtained is used as etching mask, oxygen-containing chlorine basedry etching is carried out for transferring the resist pattern to thefourth film, obtaining a pattern of the fourth film. While the fourthfilm pattern thus obtained is used as etching mask, fluorine base dryetching is carried out for transferring the fourth film pattern to thethird film, obtaining a pattern of the third film. The resist pattern isremoved at this point. A resist pattern for protecting a portion of thethird film to be left is formed on the fourth film. Further, while thethird film pattern is used as etching mask, oxygen-containing chlorinebase dry etching is carried out for transferring the third film patternto the second film, obtaining a pattern of the second film and at thesame time, removing the portion of the fourth film which is notprotected with the resist pattern. Next, while the second film patternis used as etching mask, fluorine base dry etching is carried out fortransferring the second film pattern to the halftone phase shift film todefine a pattern of the halftone phase shift film and at the same time,removing the portion of the third film which is not protected with theresist pattern. The resist pattern is removed in a conventional manner.Finally, oxygen-containing chlorine base dry etching is carried out toremove the portion of the second film where the third film has beenremoved and the portion of the fourth film where the resist pattern hasbeen removed, yielding a halftone phase shift mask.

In this way, from the halftone phase shift mask blank comprising ahalftone phase shift film a halftone phase shift mask comprising atransparent substrate and a mask pattern of the halftone phase shiftfilm can be produced.

In a photolithographic method for forming a pattern with a half pitch ofup to 50 nm, typically up to 30 nm, and more typically up to 20 nm in aprocessable substrate, comprising the steps of forming a photoresistfilm on the processable substrate and exposing the photoresist film toArF excimer laser (193 nm), through a patterned mask for transferringthe pattern to the photoresist film, the halftone phase shift mask ofthe invention is best suited for use in the exposure step.

The halftone phase shift mask obtained from the mask blank isadvantageously applicable to the pattern forming process comprisingprojecting light to the mask pattern including the pattern of halftonephase shift film for transferring the mask pattern to an object(photoresist film) on the processable substrate. The irradiation ofexposure light may be either dry exposure or immersion exposure. Thehalftone phase shift mask of the invention is effective particularlywhen a wafer of at least 300 mm as the processable substrate is exposedto a photomask pattern of light by the immersion lithography with thetendency that a cumulative irradiation energy dose increases within arelatively short time in commercial scale microfabrication.

EXAMPLE

Examples are given below for further illustrating the invention althoughthe invention is not limited thereto.

Example 1

In a DC magnetron sputtering system, a 6025 quartz substrate of 154mm×154 mm×6.35 mm thick was placed, with its receptive surface kepthorizontal. Argon and nitrogen gases were fed as the sputtering gas. Twosilicon targets having a sputter surface of diameter 170 mm were used asthe target, with the sputter surfaces kept horizontal. While thesubstrate was rotated on its axis of rotation (extending perpendicularthereto at its center) in a horizontal direction, reactive sputteringwas carried out. A halftone phase shift film consisting of silicon andnitrogen (SiN film) was deposited on the substrate, yielding a halftonephase shift mask blank.

It is assumed that the substrate has a rotational axis and a verticalline extends perpendicular to the sputter surface of each silicon targetat its center. The distances between the rotational axis and thevertical lines through two silicon targets were set identical and 375mm. Two silicon targets were arranged such that the angle includedbetween a normal line extending from the rotational axis to the verticalline through one silicon target and a normal line extending from therotational axis to the vertical line through the other silicon targetwas 144°. The flow rate of sputtering gas included 30 sccm of argon gasand 51 sccm of nitrogen gas. A power of 1,710 W was applied to onesilicon target and a power of 190 W was applied to the other silicontarget. Sputter deposition was carried out until a film thickness of 63nm was reached. The sputtering conditions corresponded to the transitionmode for each of the silicon targets.

The halftone phase shift film thus deposited was measured for phaseshift and transmittance with respect to ArF excimer laser (wavelength193 nm), over a region extending 95 mm diagonally from the center offilm surface, finding a phase shift of 176.9 to 177.5°, a median being177.2°, a difference between maximum and minimum of phase shift in filmplane of 0.6°, a transmittance of 6.08 to 6.15%, a median being 6.115%,a difference between maximum and minimum of transmittance in film planeof 0.07%. Satisfactory in-plane uniformity of phase shift andtransmittance was proven.

By XPS, the film was analyzed for composition in depth direction. TheSiN film was composed of silicon and nitrogen in an atomic ratio of47:53, corresponding to the composition of an unsaturated siliconcompound having an A value in excess of 1. The composition was constantin depth direction. Because of constant composition in film thicknessdirection, the etching rate during mask pattern formation becomesconstant in film thickness direction, suggesting that thecross-sectional profile of the mask pattern is fully vertical.

Comparative Example 1

In a DC magnetron sputtering system, a 6025 quartz substrate of 154mm×154 mm×6.35 mm thick was placed, with its receptive surface kepthorizontal. Argon and nitrogen gases were fed as the sputtering gas. Onesilicon target having a sputter surface of diameter 170 mm was used asthe target, with the sputter surface kept horizontal. While thesubstrate was rotated on its axis of rotation (extending perpendicularthereto at its center) in a horizontal direction, reactive sputteringwas carried out. A halftone phase shift film consisting of silicon andnitrogen (SiN film) was deposited on the substrate, yielding a halftonephase shift mask blank.

It is assumed that the substrate has a rotational axis and a verticalline extends perpendicular to the sputter surface of the silicon targetat its center. The distance between the rotational axis and the verticalline through the silicon target was set 375 mm. The flow rate ofsputtering gas included 30 sccm of argon gas and 50 sccm of nitrogengas. A power of 1,900 W was applied to the silicon target. Sputterdeposition was carried out until a film thickness of 63 nm was reached.The sputtering conditions corresponded to the transition mode.

By XPS, the film was analyzed for composition in depth direction. TheSiN film was composed of silicon and nitrogen in an atomic ratio of47:53, corresponding to the composition of a partially nitrided siliconcompound (nitrogen/silicon atomic ratio <4/3). The composition wasconstant in depth direction. Because of constant composition in filmthickness direction, the etching rate during mask pattern formationbecomes constant in film thickness direction, suggesting that thecross-sectional profile of the mask pattern is fully vertical.

The halftone phase shift film thus deposited was measured for phaseshift and transmittance with respect to ArF excimer laser (wavelength193 nm), over a region extending 95 mm diagonally from the center offilm surface, finding a phase shift of 179.6 to 180.60, a median being180.1°, and a difference between maximum and minimum of phase shift infilm plane of 1.00, indicating satisfactory in-plane uniformity of phaseshift. The film showed a transmittance of 6.74 to 7.36%, a median being7.05%, and a difference between maximum and minimum of transmittance infilm plane of 0.62%, indicating poor in-plane uniformity oftransmittance.

Comparative Example 2

In a DC magnetron sputtering system, a 6025 quartz substrate of 154mm×154 mm×6.35 mm thick was placed, with its receptive surface kepthorizontal. Argon and nitrogen gases were fed as the sputtering gas. Onesilicon target having a sputter surface of diameter 170 mm was used asthe target, with the sputter surface kept horizontal. While thesubstrate was rotated on its axis of rotation (extending perpendicularthereto at its center) in a horizontal direction, reactive sputteringwas carried out. A halftone phase shift film consisting of silicon andnitrogen (SiN film) was deposited on the substrate, yielding a halftonephase shift mask blank.

It is assumed that the substrate has a rotational axis and a verticalline extends perpendicular to the sputter surface of the silicon targetat its center. The distance between the rotational axis and the verticalline through the silicon target was set 375 mm. By feeding 30 sccm ofargon gas and 0 sccm of nitrogen and applying a power of 1,800 W to thesilicon target, a Si layer of 10 nm thick was deposited as a firstlayer. Then by feeding 17 sccm of argon gas and 40 sccm of nitrogen andapplying a power of 1,900 W to the silicon target, a SiN layer of 62 nmthick was deposited as a second layer. There was deposited a halftonephase shift film having a total thickness of 72 nm. The sputteringconditions corresponded to the metal mode for the first layer and thereaction mode for the second layer.

The halftone phase shift film thus deposited was measured for phaseshift and transmittance with respect to ArF excimer laser (wavelength193 nm), over a region extending 95 mm diagonally from the center offilm surface, finding a phase shift of 175.8 to 177.6°, a median being176.7°, a difference between maximum and minimum of phase shift in filmplane of 1.8°, a transmittance of 6.03 to 6.12%, a median being 6.075%,and a difference between maximum and minimum of transmittance in filmplane of 0.09%, indicating satisfactory in-plane uniformity of phaseshift and transmittance.

By XPS, the second layer was analyzed for composition in depthdirection. The SiN layer was composed of 43 at % of silicon and 57 at %of nitrogen, corresponding to the composition of a saturated siliconcompound having an A value of 1. The composition in thickness directionchanged stepwise at the interface between the first and second layers.Fluorine base dry etching is typically used for etching of this halftonephase shift film. The first and second layers were evaluated for etchingrate. When the second layer (SiN layer) was etched under conditionsproviding an etching rate of 0.5 nm/sec for the first layer (Si layer),the etching rate was 0.65 nm/sec. Because of the difference of etchingrate, a halftone phase shift film composed of at least two layers havingdifferent etching rates encounters a difference in lateral etching,suggesting that the cross-sectional profile of the mask pattern is lessvertical.

Japanese Patent Application No. 2016-162620 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. An apparatus for forming a thin film to constitute a photomask blank,comprising a substrate to constitute the photomask blank, pluraltargets, a gas supply for supplying a sputtering gas containing a raregas and a nitrogen-containing gas, and means for causing electricdischarge to the plural targets at the same time, wherein the thin filmto constitute a photomask blank is formed by rotating the substrate onits axis, sputtering the plural targets, and depositing a thin film onthe substrate, the plural targets are disposed such that provided thatthe rotational axis of the substrate and a vertical line passing thecenter of a sputter surface of each of the plural targets are paralleland spaced apart a distance, one target has the closest distance betweenthe rotational axis and the vertical line, the distance between therotational axis and the vertical line of another target is 1 to 3 timesthe distance between the rotational axis and the vertical line of theone target, and the angle included between normal lines extending fromthe rotational axis to vertical lines has a maximum value of 70° to180°.
 2. The apparatus of claim 1 wherein at least two normal linesextend from the rotational axis to vertical lines, and any of the anglesincluded between adjacent normal lines is in a range of 70° to 180°. 3.The apparatus of claim 1 wherein the plural targets are silicon targets.4. The apparatus of claim 1 wherein a halftone phase shift film formedof an unsaturated silicon compound is deposited on the substrate byreactive sputtering in transition mode.